The present invention relates generally to therapeutic methods and more particularly to methods for treatment of humans exposed to bacterial endotoxin in blood circulation as a result of, e.g., Gram negative bacterial infection, accidental injection of endotoxin-contaminated fluids, translocation of endotoxin from the gut, release of endotoxin into circulation as a result of antibiotic mediated bacterial cytolysis and the like.
Gram negative bacterial endotoxin plays a central role in the pathogenesis of gram negative sepsis and septic shock, conditions which remain leading causes of morbidity and death in critically ill patients. Endotoxin interacts with inflammatory cells, releasing endogenous mediators such as cytokines, hydrolases, peptides, prostaglandins and other compounds that contribute to the pathophysiology of septic shock. While principal causes of entry of endotoxin into circulation are Gram negative bacteremia and translocation of bacteria and bacterial products from the gut, endotoxin also may enter circulation as the result of accidental injection of contaminated fluids or through release of endotoxin from bacteria lysed as a consequence of antibiotic therapy. In the recent past, therapeutic methods proposed for treatment of sepsis and septic shock have had as their focus attempts to bind endotoxin in circulation and inhibit direct and indirect release into circulation of proinflammatory substances mediated by the presence of endotoxin. Anti-endotoxin antibodies, for example, have shown promise in inhibition of endotoxin effects in humans.
A difficulty consistently encountered in developing therapeutic methods and materials for treatment of endotoxemia in humans has been the general unreliability of in vitro and even non-human in vivo test results as an indicator of human therapeutic potential. Because the effects of endotoxin in circulation are complex and involve direct and indirect responses by many cell types in the body, test results in attempts to intervene in endotoxin's effects on a particular cell type in vitro present an incomplete basis for assessment of in vivo effects. Animal studies are complicated by differences in the effect of bacterial endotoxin on different animal species and in different models with the same species. While humans are exquisitely sensitive to endotoxin, the responses of other animals vary significantly. For example, mice and rats are far more resistant to endotoxin on a weight basis than are rabbits and dogs; a life-threatening dose in rabbits would produce minimal effects in mice. Moreover, the types of effects noted are quite variable. Dogs display intestinal hemorrhages following sublethal but shock-producing doses of endotoxin while other commonly used laboratory animals do not. Mice housed at usual room temperature become hypothermic after injection of endotoxin but develop fever when housed at 30.degree. C. See, e.g., page xx in the Introduction in Cellular Biology of Endotoxin, L. Berry ed., Volume 3 in the series Handbook of Endotoxin (R. Proctor, series ed.) Elsevier, Amsterdam, 1985.
Of interest to the background of the invention are numerous reports concerning the in vivo effects of administration of endotoxin to healthy human volunteers. Martich et al., Immunobiol., 187:403-416 (1993) provides a current and detailed review of the literature addressing the effects on circulatory system constituents brought about by experimental endotoxemia in otherwise healthy humans. Noting that the responses initiated by endotoxin in humans are common to the acute inflammatory response that is part of the host reaction to tissue injury or infection, the authors maintain that administration of endotoxin serves as a unique means of evaluating inflammatory responses as well as responses specific to endotoxin. The authors also note that, while administration of endotoxin to healthy humans is not a precise model for the entirety of host responses in septic shock, it does allow investigation of the initial host inflammatory response to bacterial endotoxin.
Martich et al. note that intravenous administration of endotoxin is uniformly accompanied by a febrile response and various constitutional changes (myalgia and the like) which are attenuated by ibuprofen but not by the phosphodiesterase inhibitor, pentoxifylline. Cardiovascular responses qualitatively similar to those observed in clinical sepsis are observed in experimental endotoxemia in humans. Characteristic increases are observed in circulating cytokines such as tumor necrosis factor .alpha. (TNF), interleukin 6 (IL-6); interleukin 1.beta. (IL-1.beta.), interleukin 8 (IL-8), and granulocyte colony stimulating factor (GCSF). Inhibitory soluble receptors of TNF were also noted to rise in a characteristic pattern following increases in levels of circulating TNF. The studies reported on in Martich et al. provided observations that ibuprofen increased levels of circulating TNF and IL-6 in experimental endotoxemia and that pentoxifylline decreased circulating TNF, but not circulating IL-6.
Human experimental endotoxemia was noted to give rise to humoral inflammatory responses similar to those observed in sepsis. The fibrinolytic system is activated and levels of tissue plasminogen activator (tPA) in circulation rise, accompanied by increases in .alpha.2-plasmin inhibitor-plasmin complexes (PAP), confirming activation of plasminogen by tPA. Endotoxin administration to humans has been observed to prompt transitory leukopenia followed by rapid leukocytosis. Neutrophil degranulation occurs with attendant release of elastase (measured as elastase/.alpha.1-antitrypsin (EAA) complexes) and lactoferrin into circulation.
Martich et al. conclude that endotoxin administration to humans represents an important model of acute inflammation which reproduces many of the inflammatory events that occur during sepsis and septic shock and provides a unique means of studying host responses to an important bacterial product.
Following publication of the Martich et al. review article, the same research group reported on a study of experimental endotoxemia wherein an attempt was made to ascertain whether endotoxin administration into circulation could give rise to increased cytokine levels in the lung as measured by broncheoalveolar lavage (BAL). Boujoukos et al., J. Appl. Physiol., 74(6):3027-3033 (1993). Even when ibuprofen was co-administered to enhance endotoxin mediated levels of circulating TNF and IL-6 in humans, no increases in TNF, IL-6 or IL-8 levels were observed in BAL fluid, suggesting that cytokine responses to endotoxin in circulation were compartmentalized and did not directly involve lung tissue endothelia.
Studies of the cardiovascular disturbances in septic shock have established that shock is usually characterized by a high cardiac index (CI) and a low systemic vascular resistance index (SVRI). [Parker et al., Crit. Care. Med., 15:923-929 (1987); Rackow et al., Circ. Shock, 22:11-22 (1987); and Parker et al., Ann. Intern. Med., 100:483-490 (1984).] Of additional interest to the background of the invention are studies of experimental endotoxemia in humans which have demonstrated depression of myocardial contractility and diastolic dysfunction. [Suffredini et al., N. Eng. J. Med., 321:280-287 (1989).]
Bactericidal/Permeability-Increasing protein (BPI) is a protein isolated from the granules of mammalian polymorphonuclear neutrophils (PMNs), which are blood cells essential in the defense against invading microorganisms. Human BPI protein isolated from PMNs by acid extraction combined with either ion exchange chromatography [Elsbach, J. Biol. Chem., 254:11000 (1979)] or E. coli affinity chromatography [Weiss, et al., Blood, 69:652 (1987)] has optimal bactericidal activity against a broad spectrum of gram-negative bacteria. The molecular weight of human BPI is approximately 55,000 daltons (55 kD). The amino acid sequence of the entire human BPI protein, as well as the DNA encoding the protein, have been elucidated in FIG. 1 of Gray et al., J. Biol. Chem., 264:9505 (1989), incorporated herein by reference.
The bactericidal effect of BPI has been shown in the scientific literature published to date to be highly specific to sensitive gram-negative species, with toxicity generally lacking for other microorganisms and for eukaryotic cells. The precise mechanism by which BPI kills bacteria is not yet completely elucidated, but it is known that BPI must first attach to the surface of susceptible gram-negative bacteria. This initial binding of BPI to the bacteria involves electrostatic and hydrophobic interactions between the basic BPI protein and negatively charged sites on endotoxin. BPI binds to lipid A, the most toxic and most biologically active component of endotoxins.
In susceptible bacteria, BPI binding is thought to disrupt lipopolysaccharide (LPS) structure, leading to activation of bacterial enzymes that degrade phospholipids and peptidoglycans, altering the permeability of the cell's outer membrane, and initiating events that ultimately lead to cell death. [Elsbach and Weiss, Inflammation: Basic Principles and Clinical Correlates, eds. Gallin et al., Chapter 30, Reven Press, Ltd. (1992)]. BPI is thought to act in two stages. The first is a sublethal stage that is characterized by immediate growth arrest, permeabilization of the outer membrane and selective activation of bacterial enzymes that hydrolyze phospholipids and peptidoglycan. Bacteria at this stage can be rescued by growth in serum albumin supplemented media. The second stage, defined by growth inhibition that cannot be reversed by serum albumin, occurs after prolonged exposure of the bacteria to BPI and is characterized by extensive physiologic and structural changes, including penetration of the cytoplasmic membrane.
Permeabilization of the bacterial cell envelope to hydrophobic probes such as actinomycin D is rapid and depends upon initial binding of BPI to endotoxin, leading to organizational changes which probably result from binding to the anionic groups in the KDO region of endotoxin, which normally stabilize the outer membrane through binding of Mg.sup.++ and Ca.sup.++. Binding of BPI and subsequent bacterial killing depends, at least in part, upon the endotoxin polysaccharide chain length, with long O chain bearing organisms being more resistant to BPI bactericidal effects than short, "rough" organisms [Weiss et al., J. Clin. Invest., 65:619-628 (1980)]. This first stage of BPI action is reversible upon dissociation of the BPI from its binding site. This process requires synthesis of new LPS and the presence of rivalent cations [Weiss et al., J. Immunol., 132:3109-3115 (1984)]. Loss of bacterial viability, however, is not reversed by processes which restore the outer membrane integrity, suggesting that the bactericidal action is mediated by additional lesions induced in the target organism and which may be situated at the cytoplasmic membrane [Mannion et al., J. Clin. Invest., 86:631-641 (1990)]. Specific investigation of this possibility has shown that on a molar basis BPI is at least as inhibitory of cytoplasmic membrane vesicle function as polymyxin B [In't Veld et al., Infection and Immunity, 56:1203-1208 (1988)] but the exact mechanism has not yet been elucidated.
A proteolytic fragment corresponding to the N-terminal portion of human BPI holoprotein possesses essentialy all the bactericidal efficacy of the naturally-derived 55 kD human holoprotein. [Ooi et al., J. Bio. Chem., 262:14891-14894 (1987)]. In contrast to the N-terminal portion, the C-terminal region of the isolated human BPI protein displays only slightly detectable anti-bacterial activity. [Ooi et al., J. Exp. Med., 174:649 (1991).] A BPI N-terminal fragment, comprising approximately the first 199 amino acid residues of the human BPI holoprotein and referred to as "rBPI.sub.23," has been produced by recombinant means as a 23 kD protein. [Gazzano-Santoro et al., Infect. Immun., 60:4754-4761 (1992).]
Of additional interest to the present application are the disclosures of references which relate to the potentialion of BPI bactericidal activity by 15 kD proteins derived from the granules of rabbit PMNs designated p15. Ooi et al., J. Biol. Chem., 265:15956 (1990), disclose two related 15 kD proteins derived from rabbit PMN granules which potentiate the first sublethal stage of BPI antibacterial activity but have an inhibitory effect on the second lethal stage of BPI antibacterial activity. Levy et al., J. Biol. Chem., 268:6058-6063 (1993), disclose the sequences of cDNAs encoding the two rabbit proteins and report that the protein with the stronger potentiating effect reduces the required dose of BPI for the early bacteriostatic effect by about 20-fold.
Of particular interest to the background of the present invention are reports of interaction between bacterial endotoxin and BPI protein products in various in vitro and non-human in vivo assay systems. As one example, Leach et al., Keystone Symposia "Recognition of Endotoxin in Biologic Systems", Lake Tahoe, Calif., Mar. 1-7, 1992 (Abstract) reported that rBPI.sub.23 (as described in Gazzano-Santoro et al., supra) prevented lethal endotoxemia in actinomycin D-sensitized CD-1 mice challenged with E. coli 011:B4 LPS. In additional studies Kohn et al., J. Infectious Diseases, 168:1307-1310 (1993) demonstrated that rBPI.sub.23 not only protected actinomycin-D sensitized mice in a dose-dependent manner from the lethal effects of LPS challenge but also attenuated the LPS-induced elevation of TNF and IL-1 in serum. Ammons et al., [Circulatory Shock, 41: 176-184 (1993)] demonstrated in a rat endotoxemia model that rBPI.sub.23 produced a dose-dependent inhibition of hemodynamic changes associated with endotoxemia. Kelly et al., Surgery, 114: 140-146 (1993) showed that rBPI.sub.23 conferred significantly greater protection from death than an antiendotoxin monoclonal antibody (E5) in mice innoculated intratracheally with a lethal dose of E. coli. Kung, et al., International Conference on Endotoxin Amsterdam IV, Aug. 17-20 (1993) (Abstract) disclosed the efficacy of rBPI.sub.23 in several animal models including live bacterial challenge and endotoxemia models.
M. N. Marra and R. W. Scott and co-workers have addressed endotoxin interactions with BPI protein products in U.S. Pat. Nos. 5,089,274 and 5,171,739, in published PCT Application WO 92/03535 and in Matra et al., J. Immunol., 144:662-665 (1990) and Marra et al., J. Immunol., 148:532-537 (1992). In vitro and non-human in vivo experimental procedures reported in these documents include positive assessments of the ability of BPI-containing granulocyte extracts, highly purified granulocytic BPI and recombinant BPI to inhibit endotoxin stimulation of cultures of human adherent mononuclear cells to produce tumor necrosis factor .alpha. (TNF) when endotoxin is pre-incubated with the BPI product. Pre-incubation of endotoxin with BPI protein products was also shown to diminish the capacity of endotoxin to stimulate (upregulate) neutrophil cell surface expression of receptors for the complement system components C3b and C3bi in vitro. However, neither of these complement system components is known to have been demonstrated to be present in increased amounts in circulation as a result of the presence of endotoxin in human circulation. The experimental studies reported in these documents included in vivo assessments of endotoxin interaction with BPI protein products in test subject mice and rats. In one series of experiments, BPI was noted to inhibit stimulation of lung cell production of TNF (measured on the basis of cytotoxicity to fibrosarcoma cells of broncheoalveolar lavage fluids) in mice challenged by intranasal administration of endotoxin. In another series of experiments, administration of BPI was noted to protect mice and rats from lethal challenge with various bacterial endotoxin preparations and live Pseudomonas and binding of BPI to endotoxin was noted to diminish pyrogenicity in rabbits. As noted above, however, Boujoukas et al., J. Appl. Physiol., 74(6):3027-3033 (1993) have demonstrated that, while administration of endotoxin to human circulation resulted in increased levels of circulating TNF, IL-6 and IL-8, no increases in these substances could be detected in broncheoalveolar lavage fluids of the human subjects.
Since the filing of parent U.S. patent application Ser. No. 08/188,221 on Jan. 24, 1994, additional studies of in vitro and in vivo effects of BPI have been published. Fisher et al., Critical Care Med., 22(4):553-558 (1994) addressed studies in mice, rats and rabbits and concluded that, "The exciting possibility that bactericidal/permeability-increasing protein may be a specific therapeutic agent to enhance the natural negative feedback mechanisms for regulating endotoxin in humans is worth investigation." Marra et al., Critical Care Med., 22(4):559-565 (1994) addressed studies in mice and concluded that, "The potent endotoxin-binding and -neutralizing properties of bactericidal/permeability-increasing protein indicate that it might be useful in the treatment of endotoxin-related disorders in humans."
Thus, while BPI protein products have been established to have potentially beneficial interactions with endotoxin in a variety of in vitro and non-human in vivo model systems, nothing is known concerning effects of these products in humans actually exposed to bacterial endotoxin in circulation as a result of, e.g., Gram negative bacterial infection, treatment with antibiotics, accidental injection with endotoxin-contaminated fluids, translocation of endotoxin from the gut, and the like.